Probe Diagnostics of Expanding Plasmas at Low Gas Pressure

Abstract

Summary: Results from tandem‐type probe diagnostics of a plasma source based on an inductive discharge are presented in this study. The driver region is in the classical form of a cylindrically shaped inductive discharge, with a coil positioned over a gas discharge tube, whereas a bigger metal chamber provides volume for plasma expansion. Low pressure argon discharges were studied. The axial profiles of the plasma parameters were measured in the discharge in the metal chamber. The results obtained show that decreasing electron temperature and plasma density with increasing distance from the driver characterizes the behavior of the expanding plasmas. Moreover, two regions with different rates of variation of the plasma parameters complete the plasma expansion volume: a faster drop close to the driver and slow axial changes away from it. The gas pressure and power applied for the discharge maintenance were the external parameters varied.

Expanding Plasma Region of an Inductively Driven Hydrogen Discharge

“…Three regions complete the axial profile of the density: 1) a fast drop of n e in the region (z ≤ 2 cm) close to the driver; 2) a plateau region for 2 cm ≤ z ≤ 6 cm; and 3) a slow decrease for z ≥ 6 cm. Similar structuring of the axial profile of the electron density has also been observed before in argon discharges [16], [17]. Increasing applied power leads to a density increase by keeping the trends of the axial variation.…”

“…4) is the main conclusion from the experiment that was also made before but for argon discharges [17]. The very weak axial variation of T e away from the driver calls for the importance of nonlocality that is associated with thermal conductivity effects.…”

“…1) is the same as has been described before [17], [18]: an inductively driven plasma source with two parts of the discharge vessel. The active part of the discharge (its driver region) is a cylindrical type of inductive discharge produced by the RF current in a coil positioned around a quartz tube (with internal and external diameters d 1 = 4.5 cm and d 2 = 4.9 cm, respectively, and a length of l = 30 cm).…”

“…Complicated ion dynamics and ion temperature anisotropy (higher perpendicular ion temperature than the parallel one, with both being defined with respect to the magnetic field) arise when the plasma expands into a volume with a strongenough external magnetic field [8], [30]- [32]. In addition, results from recent experiments [17] on plasma expansion in an inductively driven source in argon gas show that the higher pressure range-about 10 mtorr-possesses a peculiar behavior even when an external magnetic field is not applied to the plasma expansion volume: The electron temperature drops fast away from the driver, and regions of different rates of the axial decrease of the plasma density are formed in the expanding plasma volume. This brings up the question of whether such a behavior is a general characteristic-for discharges in different gases-of the expanding plasmas when the gas pressure is in the range of tens of militorrs.…”

“…The difficulties in obtaining the plasma parameters from probe characteristics that were recorded from hydrogen discharges stem from the complexity of the ion composition in the discharge (three types of positive ions H + , H + 2 , and H + 3 as well as negative ions H − ). In this connection, the electron density is determined from the electron energy distribution function (EEDF) and not from the ion saturation current of the probe characteristics, as has been done before [16], [17] in argon discharges. Spatial variation of the plasma parameters-electron temperature and density as well as plasma and floating potentials-completes the set of obtained results.…”